This invention relates to a focus servo apparatus for automatic focus control of the pick-up of a device such as an optical disk drive.
In an optical disk drive, a focus servo apparatus is used to maintain a constant distance between an objective lens in the pick-up and the surface of the disk, so that the objective lens will focus a laser beam accurately onto the information-carrying surface of the disk. The focus servo apparatus includes an automatic control loop called a servo loop. Before the servo loop can operate, the object lens must be within a certain controllable range, called the lead-in range. Accordingly, when the drive begins to operate, the focus servo apparatus must first move the objective lens into the lead-in range; then the servo loop can take over, move the objective lens to the correct-focus position, and maintain it in that position.
Focus servo apparatus of this type has been disclosed in Japanese Patent Applications Laid-open No. 1984/171034, 1985/629, and 1986/206928. The structure of these devices will be described with reference to the block diagram in FIG. 9.
In FIG. 9, the disk 1 comprises a substrate of a material such as glass or plastic overlain by an information-carrying medium. The pick-up 2 comprises an objective lens 2a, as well as a semiconductor laser, a photodetector, other optical components, and an actuator which are not shown. The pick-up 2 is connected to a servo loop SL and a lead-in circuit LI.
In the servo loop SL, a focus error detector 3 receives the output from the photodetector in the pick-up 2, detects deviation from the correctly focused state, and generates a focus error signal 3S. The focus error signal 3S is fed through a phase compensation circuit 4 and a servo loop switch 5 to a driver circuit 6. The purpose of the phase compensation circuit 4 is to stabilize the operation of the servo loop. The driver circuit 6 drives the actuator in the pick-up 2, causing it to move the objective lens in a direction substantially perpendicular to the disk 1. The servo loop SL is designed to move the objective lens 2a in such a way as to reduce the focus error to zero. It thus causes the objective lens 2a to follow variations in the height of the disk surface and maintain a constant distance from the disk 1.
The focus error signal 3S is also supplied to a lead-in detection circuit 7, which determines whether the objective lens 2a is within the lead-in range. The lead-in detection circuit 7 generates an acquisition signal 7S that drives the servo loop switch 5 to close the servo loop when the objective lens 2a is within the lead-in range. When closed, the servo loop SL operates as described above to maintain the objective lens 2a at the correct distance from the disk surface.
When the objective lens 2a is not within the lead-in range, the signal 7S from the lead-in detection circuit 7 places the servo loop switch 5 in a state in which the servo loop is opened and does not control the movement of the objective lens 2a. In this state the driver 6 is connected through the servo loop switch 5 to the focus servo lead-in circuit LI.
The function of the servo lead-in circuit LI connected to the servo loop switch 5 is to bring the objective lens 2a into the lead-in range when the disk drive starts to operate. The servo lead-in circuit LI comprises a control circuit 8, and a drive signal generating circuit 9 connected between the control circuit 8 and the servo loop switch 5. The control circuit 8 generates a lead-in signal 8S, from which the drive signal generating circuit 9 generates a drive signal 9S for driving the objective lens. The drive signal generating circuit 9 is an integrating circuit comprising an amplifier 10, a resistor 11, and a capacitor 12.
The lead-in operation of the focus servo apparatus in FIG. 9 will be explained with reference to FIG. 10 and FIG. 11. FIG. 10 indicates how the focus error signal 3S in FIG. 9 depends on distance from the disk. FIG. 11 is a timing chart of the lead-in operation.
The lead-in range is the range between the maximum and minimum values of the focus error signal 3S in FIG. 10. In general, the lead-in range is quite narrow, on the order of 10 .mu.m to 20 .mu.m. When the disk drive starts operating the objective lens 2a is distant from the disk surface and outside the lead-in range, so the servo loop switch 5 is in the state in which servo loop is open and the driver circuit 6 is connected to the lead-in circuit. As indicated in FIG. 11, the control circuit 8 sends a lead-in signal 8S to the drive signal generating circuit 9, causing it to generate a drive signal 9S which is routed by the servo loop switch 5 to the driver circuit 6. In response, the driver circuit 6 begins driving the objective lens 2a toward the surface of the disk 1. When the lead-in detection circuit 7 detects that the distance between the disk 1 and the objective lens 2a is within the lead-in range indicated in FIG. 10, it generates an acquisition signal 7S which causes the servo loop switch 5 to disconnect the driver circuit 6 from the lead-in circuit LI and close the servo loop. The servo loop then brings the objective lens 2a to the in-focus position marked 0 in FIG. 10 and maintains it there as already described, following variations in the height of the surface of the disk 1.
The configuration of the drive signal generating circuit 9 in FIG. 9 is that shown in Japanese Patent Application Laid-open No. 1985/629 or 1986/206928. Because this circuit integrates the lead-in signal 8S, the drive signal 9S increases in magnitude with time. The reason for the use of this type of circuit is that in the prior art the objective lens actuator is supported by or interacts with a spring which resists the movement of the objective lens with a force that increase in proportion to the distance from the initial position. The increasing magnitude of the drive signal 9S balances the increasing resistance of the spring to drive the objective lens at a substantially constant speed. To ensure that the speed does not become excessive, the time constant of the drive signal generating circuit 9 is fairly large. Depending on this time constant and the strength of the spring, however, the objective lens 2a may acquire such a high speed that a large transient response occurs when the focus servo loop is closed. If this response moves the objective lens 2a back outside the lead-in range, the lead-in operation fails. To overcome this problem and stabilize the servo lead-in operation, the lead-in circuit of Japanese Patent Laid-open No. 1984/171034 included a means of switching the integrating time constant.
Because the focus servo apparatus of the prior art was designed as described above, two problems occur if, as in the inventor's previous Japanese Patent Application No. 1985/152381, the objective lens actuator has a sliding support and does not interact with a spring. The first problem is that without a spring to counteract the increasing drive signal 9S, the objective lens 2a gradually accelerates, which increases the likelihood that the lead-in operation will fail.
The second problem is that the sliding support generates a friction force f. As shown in FIG. 12, this friction force has a high initial value fmax. If the driving force does not initially exceed the initial fmax, the objective lens 2a at first does not move; then when the driving force increases sufficiently to overcome fmax and the objective lens does begin to move, the friction f decreases. The result is a sudden, rapid acceleration of the objective lens 2a, which tends to make the lead-in operation unstable.
A further problem with the focus servo apparatus described above is that optical drive devices are mounted in both the position shown in FIG. 13A, in which the disk is horizontal, and the position shown in FIG. 13B, in which the disk is vertical. When the optical disk drive is mounted so that the disk is horizontal, if the pick-up 2 is located below the disk 1, the weight of the objective lens and the movable part of its actuator becomes an additional load that the drive signal 9S must overcome. When the optical disk drive is mounted so that the disk is vertical, the force of gravity acts at a direction substantially perpendicular to the direction of movement of the objective lens and actuator, so their weight is absorbed by the actuator support and does not act as a load to the actuator. To provide for these different mounting orientations, it is therefore necessary to change the magnitude of the drive signal 9S, which detracts from the convenience of use of the drive. Moreover, in the horizontal orientation a steady-state error persists during the focus following operation even after the lead-in operation is completed.